In the prior art, various methods and apparatus have been proposed to destroy or dispose of polychlorinated biphenyls, commonly known as PCBs.
Prior to the recognition that PCBs posed a serious threat to the environment, these compounds were utilized in a wide range of applications as a result of their very high stability, in particular, at high temperatures. Although PCBs were used in coatings, lubricants, pumping fluids, their main utility was as dielectric fluids in transformers and capacitors.
As a result of their high stability and extremely slow biodegradation rates, PCBs that enter the environment either in soil or fluids pose a substantial environmental hazard. As a result of the threat posed to the environment by PCBs, legislation has been enacted restricting the use and application thereof as well as providing for eventual disposal.
One method of treating materials containing PCBs is high temperature incineration, with subsequent treatment of the products of combustion to remove hydrogen chloride produced from the breakdown of the PCBs. However, these incineration processes are not 100 percent effective which results in PCBs or equivalent toxic substances being emitted into the atmosphere.
An alternative method of degradation of PCBs is disclosed in U.S. Pat. No. 4,931,167 to Wilwerding. In this method, polychlorinated biphenyls are chemically degraded by contact with a Lewis acid catalyst in a non-aqueous liquid medium, in the presence of a cation which combines with the chlorines on the PCBs to form a solid chloride of the cation which will precipitate from the liquid medium. In this process, the PCB-containing material must have all water removed prior to the degradation process. Moreover, the process is performed using an alcohol solvent and a metal halide Lewis acid such as a metal chloride, metal bromide or a combination thereof.
Another prior art method proposed to dispose of polychlorinated biphenyls is disclosed in U.S. Pat. No. 4,469,661 to Shultz. In this method, solids containing the hazardous halogenated hydrocarbons are subjected to an extraction step to separate the PCBs from the soil or other contaminated material. After the extraction step, the high-boiling hydrocarbon solvent and PCB are vaporized and injected into a molten aluminum bath to form aluminum chloride and destroy the PCBs. European Patent Application Number 0 170 714 to Shultz discloses a similar process as described above for the Shultz patent.
U.S. Pat. No. 3,640,821 to Sweeny et al. discloses a method of reductive degradation of halogenated pesticides, typically DDT, by reacting the pesticides with metallic zinc under mildly acid conditions to produce less highly halogenated degradation products of lower toxicity. In this method, DDT is reacted with an aqueous suspension of finely divided metallic zinc at a pH below about 4 for a period of time sufficient to convert a major portion of the DDT to DDEt. The Sweeny et al. patent indicates that only the aliphatic halogens are removed using their process and a complete dehalogenation of compounds such as DDT is extremely difficult.
Disadvantages associated with these prior art processes include the need for extensive chemical process equipment to effect the degradation or destruction of the PCBs. In the patent to Wilwerding, various mixing and reaction vessels are required to perform the extracting, vaporizing and reacting steps of the process. The Shultz process requires extensive equipment to prepare the PCBs prior to contact with the molten aluminum. In known incineration processes, extensive equipment is required for both the incineration aspect and waste gas treatment aspects of these processes.
In response to these recognized deficiencies in prior art processes, a need has developed to provide an improved in-situ method of disposing of or treating PCB-contaminated soil.
In response to this need, the present invention provides apparatus and a method of treating PCB-contaminated soil which effectively dechlorinates the PCB content of the soil in situ without endangering the surrounding environment.
None of the prior art discussed above teaches or fairly suggests an in-situ process of treating PCB-contaminated soil which results in high levels of reduction in PCBs over a period of time.